OZONE PRECURSOR LEVELS AND RESPONSES TO EMISSIONS REDUCTIONS - ANALYSIS OF REGIONAL OXIDANT MODEL RESULTS

An analysis of results from the Regional Oxidant Modeling for Northeas t Transport (ROMNET) study (U.S. EPA, 1991, EPA-450/4-91-002a) has inv estigated the chemical conditions under which air quality was predicte d to improve with reductions in ROG and/or NOx emissions, or with chan ges in the composition of ROG emissions. The ROMNET simulations used e missions projected to the year 2005, with meteorological conditions fr om July 1988. Predicted concentrations of PAN, HNO3, H2O2 and HCHO are shown along with O-3 for the 2005 base case, allowing limited compari sons to be made with field observations and results from other modelin g studies. Predicted secondary pollutant concentrations indicate an un usual degree of photochemical activity over much of the model domain, directionally consistent with the extreme nature of the July 1988 epis ode. Reducing NOx emissions was predicted to reduce O-3 in grid cells in which reactive nitrogen (NOy) concentrations were below about 25 pp b, but to be counterproductive for some cells with higher NOy. The New York City area where NOx control was predicted to be counterproductiv e was characterized by very high NOx to NOy ratios. Ozone was relative ly insensitive to ROG controls in grid cells with NOy concentrations b elow 5-10 ppb. Comparison of unweighted ROG concentrations with concen trations weighted by HO rate constants (i.e. reactivity) showed that t he latter varied less across locations. Predicted spatial gradients of NOy were generally sharper than those of reactivity-weighted ROG, sup porting a dominant role for variations in NOy in controlling the sensi tivity of ozone to its precursors. Reductions in reactivity-weighted R OG achieved with composition changes were similar to reductions achiev ed with ROG emissions cuts, explaining the similar response of ozone t o these two control strategies.